(1) Field of the Invention
The present invention relates generally to communications systems and, more particularly, to a high performance iterative and adaptive decision feedback equalizer which is especially suitable for use in underwater telemetry.
(2) Description of the Prior Art
The underwater environment provides numerous difficult obstacles for acoustic communications. The ocean acoustic channel produces large amplitude and phase fluctuations on acoustic signals transmitted therethrough causing temporal, spatial, and frequency dependent fluctuations. Multipath distortion is a significant problem. Underwater regions often experience high and/or variable sound attenuation. Ambient ocean noise influences the received signal-to-noise ratio and may require high transmission power levels to achieve suitable ratios depending on the conditions.
Presently utilized underwater coherent acoustic telemetry systems are often able to transmit M-ary Phase Shift Keying (MPSK) and M-ary Quadrature Amplitude Modulation (MQAM) signals. At the receiver end, these coherent signals may be processed by an adaptive multi-channel decision feedback equalizer (DFE). The DFE is then usually followed by a de-interleaver and an error correction decoder operating in a single pass fashion. The de-interleaver randomizes the errors and the error correction decoder tries to correct these randomly distributed errors. The error correction decoder is usually a Viterbi decoder for a convolutional code. The overall performance obtained by this type of prior art underwater telemetry system is often acceptable, but is not satisfactory in many situations. The desire for performance improvement has led to higher performance algorithms whose complexity is orders of magnitude greater than the standard decision feedback equalizer (DFE) system followed by de-interleaving and decoding. The turbo-equalization algorithm is one such algorithm that has performed much better than the normal algorithm but the cost has been extremely high complexity.
Turbo equalization and turbo coding may be applied to many detection and decoding problems. Turbo coding involves concatenation of simple component codes with an interleaver so that decoding can be performed in steps using algorithms of manageable complexity. However, the complexity of prior art turbo equalization increases exponentially with the number of channels and/or other factors, thereby making a multichannel telemetry system, as is typically utilized in underwater telemetry systems, highly complex. More particularly, the complexity of the prior art turbo-equalizer grows with channel complexity, modulation level, and spatial and/or time diversity. The complexity of a prior art turbo-equalizer is therefore orders of magnitude greater than the typical DFE structure discussed above.
The following U.S. Patents describe various prior art systems that may be related to the above and/or other telemetry systems:
U.S. Pat. No. 5,301,167, issued Apr. 5, 1994, to Proakis et al., discloses an underwater acoustic communications system that utilizes phase coherent modulation and demodulation in which high data rates are achieved through the use of rapid Doppler removal, a specialized sample timing control technique and decision feedback equalization including feedforward and feedback equalizers. The combined use of these techniques dramatically increases data rates by one and sometimes two orders of magnitude over traditional FSK systems by successfully combating fading and multipath problems associated with a rapidly changing underwater acoustic channel that produce intersymbol interference and makes timing optimization for the sampling of incoming data impossible.
U.S. Pat. No. 5,559,757, issued Sep. 24, 1996, to Catipovic et al., discloses an underwater acoustic telemetry system that uses spatially distributed receivers with aperture sizes from 0.35 to 20 m. Output from each receiver is assigned a quality measure based on the estimated error rate, and the data, weighted by the quality measure, is combined and decoded. The quality measure is derived from a Viterbi error-correction decoder operating on each receiver. The quality estimator exploits the signal and noise differential travel times to individual sensors. The spatial coherence structure of the shallow-water acoustic channel shows relatively low signal coherence at separations as short as 0.35 m. Increasing receiver spacing beyond 5 m offers additional benefits in the presence of impulsive noise and larger scale inhomogeneities in the acoustic field. Diversity combining, even with only two receivers, can lower uncoded error rates by up to several orders of magnitude while providing immunity to transducer jamming or failure.
U.S. Pat. No. 6,295,312 B1, issued Sep. 25, 2001, to Susan M. Jarvis, discloses a method and system for communicating in a time-varying medium. A transmitter sends transmissions of the same message data separated in time with respect to one another. A single sensor receives the transmissions. Each received transmission is buffered until all of the transmissions that were sent are received. The buffered transmissions are simultaneously processed via multichannel adaptive equalization only when all of the transmissions that were sent are received.
The above cited prior art does not disclose a system whose complexity is similar to that of the prior art decision feedback equalizer followed by a de-interleaver and an error correction decoder, but whose performance is greatly improved. The above cited prior art also does not disclose decision feedback equalizers utilizing hard and/or soft feedback from the decoder. The solutions to the above described and/or related problems have been long sought without success. Consequently, those skilled in the art will appreciate the present invention that addresses the above and other problems.
It is a general purpose of the present invention to provide an improved telemetry system.
Yet another object is to provide an augmented high performance iterative receiver algorithm for underwater acoustic telemetry.
It is another object of the present invention to provide a hard-iterative DFE structure and a soft-iterative DFE structure that is superior to the standard DFE structure.
It is yet another object of the present invention to provide a system which has linear complexity growth with the size of the symboling constellation as opposed to more complex systems such as turbo-equalization which experience exponential complexity growth.
An advantage of the present invention is that it takes advantage of the attractive features of the DFE structure such as diversity combining, modest complexity increase with channel complexity, symbol synchronization, and phase tracking while providing higher performance than a standard DFE with less complexity than the turbo-equalizer.
A feature of one embodiment of the invention combines a decision feedback adaptive equalizer (DFE) with a turbo-equalizer whereby the decision feedback equalizer or variant thereof provides a pre-processing stage for a turbo-equalizer that significantly limits the complexity of the turbo-equalizer.
An advantage of the present invention is superior performance as compared to the standard DFE structure.
Another advantage is that time or spatial signal diversity can be processed with low complexity within the DFE to provide a single stream of diversity combined symbols which can be processed with a simplified turbo-equalizer construction for use in multichannel transmissions.
Yet another advantage of the present invention is that a DFE structure may be utilized therein to take advantage of fractional spacing to help synchronize symbols.
Yet another advantage of the present invention is that a DFE structure may be utilized to reduce the extent of the channel response and therefore allow a turbo-equalizer to operate on a much shorter impulse response in order to reduce the complexity thereof.
These and other objects, features, and advantages of the present invention will become apparent from the drawings, the descriptions given herein, and the appended claims. However, it will be understood that above listed objects and advantages of the invention are intended only as an aid in understanding certain aspects of the invention, are not intended to limit the invention in any way, and do not form a comprehensive or exclusive list of objects, features, and advantages. Accordingly, the present receiver is operable for use in a telemetry system such as an underwater telemetry system and may comprise one or more elements such as, for instance, at least one data input channel connected to the receiver, and a decision feedback equalizer for receiving the data input channel. The present receiver preferably produces an estimated symbol sequence output during a plurality of iterations of operation. The present receiver may further comprise a symbol-by-symbol detector which is preferably operable for receiving the estimated symbol sequence output and operable to produce a symbol-by-symbol detector output. A decoder is provided for receiving the estimated symbol sequence output and for producing a decoded output. An iterative feedback connection is provided between the decoder and the decision feedback equalizer to provide feedback from the decoder for use in at least some of the plurality of iterations of operation of the decision feedback equalizer. In a preferred embodiment, the decoder may be a Viterbi decoder or a MAP decoder.
The receiver further may comprise a feedback filter for the decision feedback equalizer and in one embodiment may comprise a switch between the symbol-by-symbol detector and the feedback filter and the iterative feedback connection operable for selectively connecting the symbol-by-symbol detector output to the feedback filter or for connecting the iterative feedback connection to the feedback filter. In this embodiment, the switch is operable for connecting the symbol-by-symbol detector output to the feedback filter during a first iteration of the plurality of iterations and then connecting the iterative feedback connection to the feedback filter for subsequent of the plurality of iterations, at least until a stop criterion is reached.
The receiver may further comprise a feedback filter wherein the feedback filter is operable for receiving hard values of decoded symbols from the decoder by means of the iterative feedback connection.
In another embodiment the iterative feedback connection between the decoder and the decision feedback equalizer may connect to the symbol-by-symbol detector. The iterative feedback connection provides log likelihood ratio information and the symbol-by-symbol detector may further comprise a converter for converting estimated symbol sequence output from said decision feedback equalizer to log likelihood ratio information. A combiner may be utilized to combine the log likelihood ratio information from the iterative feedback connection and the log likelihood ratio information produced by the converter. The symbol-by-symbol detector further comprises a decision module for receiving the combiner output and producing hard values of decoded symbols for the feedback filter.
A method of operation is provided which may comprise one or more steps such as, for instance, iteratively processing a received signal with a decision feedback equalizer to produce estimated symbol sequence information and post-processing the estimated symbol sequence information with a decoder wherein the decoder may comprise at least a Viterbi decoder or a MAP decoder. Other steps may comprise providing a feedback connection between the decoder and the decision feedback equalizer to provide feedback information from the decoder for use in at least some plurality of iterations of the processing by the decision feedback equalizer.
The method may further comprise selectively utilizing the feedback information from the decoder so that after a first iteration of processing by the decision feedback equalizer, then the feedback information is utilized in subsequent of the plurality of iterations of the processing, at least until a stop criterion is reached.
In one possible embodiment, the method may comprise controlling a switch for connecting the feedback connection to the feedback filter in the decision feedback equalizer.
In another possible embodiment, the method may comprise combining the estimated symbol sequence information with log likelihood ratio information produced utilizing the decoder. The method may comprise processing the estimated symbol sequence information prior to the step of combining by converting the estimated symbol sequence information to log likelihood ratio information. The step of converting may further comprise multiplying the estimated symbol sequence by a factor wherein the factor comprises computing a variance of the estimated symbol sequence.
The method may comprise iteratively processing BPSK modulated signals or may comprise iteratively processing MPSK and MQAM modulated signals and/or may be utilized for other types of modulated signals, as desired.